The text defines personality as the composite of physical, mental, emotional, and temperamental traits and examines how the internal secretion (endocrine) glands contribute to those traits. It reviews early research that stimulated scientific study of ductless glands, describes experimental and clinical methods used to link glandular activity to behavior, and outlines the nature and functions of major glands. It discusses how abnormal size or activity of specific glands can alter growth, temperament, and mental functioning, and considers clinical implications for diagnosis, treatment, and understanding variations within normal personality.